1. Field of the Invention
The present invention relates generally to semiconductor manufacturing and, more particularly, to the characterization and control of lithographic process conditions used in microelectronics manufacturing.
2. Description of Related Art
During microelectronics manufacturing, a semiconductor wafer is processed through a series of tools that perform lithographic processing, usually followed by etch or implant processing, to form features and devices in the substrate of the wafer. Such processing has a broad range of industrial applications, including the manufacture of semiconductors, flat-panel displays, micromachines, and disk heads.
The lithographic process allows for a mask or reticle pattern to be transferred via spatially modulated light (the aerial image) to a photoresist (hereinafter, also referred to interchangeably as resist) film on a substrate. Those segments of the absorbed aerial image, whose energy (so-called actinic energy) exceeds a threshold energy of chemical bonds in the photoactive component (PAC) of the photoresist material, create a latent image in the resist. In some resist systems the latent image is formed directly by the PAC; in others (so-called acid catalyzed photoresists), the photo-chemical interaction first generates acids which react with other photoresist components during a post-exposure bake to form the latent image. In either case, the latent image marks the volume of resist material that either is removed during the development process (in the case of positive photoresist) or remains after development (in the case of negative photoresist) to create a three-dimensional pattern in the resist film. In subsequent etch processing, the resulting resist film pattern is used to transfer the patterned openings in the resist to form an etched pattern in the underlying substrate. It is crucial to be able to monitor the fidelity of the patterns formed by both the photolithographic process and etch process, and then to control or adjust those processes to correct any deficiencies.
Lithographic systems replicate circuit patterns by projecting the image of a mask pattern onto a wafer, and consist of imaging tools that expose patterns and processing tools that coat, bake and develop the substrates. The pattern may consist of features of varying size and density, all of which must be printed simultaneously with dimensional fidelity to design. As used herein, the term critical dimension (CD) or critical width refers to the smallest dimension of a pattern or feature that can be produced by the lithographic system.
The dose setting on the imaging tool determines the average energy in the aerial image. Optimum dose produces energy equal to the resist threshold at the desired locations on the pattern. The focus setting on the imaging tool determines the average spatial modulation in the aerial image. Optimum focus produces the maximum modulation in the image. The settings of many other imaging and processing tool parameters determine the “effective” dose and defocus (deviation from optimum focus) that form the latent image in the resist film. Dimensional fidelity depends primarily on the control of these two image attributes: 1) the average energy in the image determined by dose and 2) the modulation in the image determined by focus.
To achieve optimum dimensional control the image must be brought into focus on the wafer surface at a dose corresponding to the desired pattern dimensions. This requires both that the wafer be positioned in the focal plane of the projection lens and that the focal plane be well defined and stable. Focus error has been found to have two distinct characteristics: a) defocus, where the focal plane is displaced from the desired surface, and b) blur, where the focal plane is ill-defined. While defocus and blur can have similar deleterious effects on the quality of the printed image, their cause and means of control are different.
Defocus error is shown in FIG. 1, where light energy from a source 20 is focused by a lens 22 to a focal plane 26a, which is displaced by distance D from wafer surface 24. In step and scan lithography, causes of defocus include focus system error, tilts along the scan and slit, wafer non-flatness, lens aberrations such as field curvature and astigmatism, and low-frequency vibration (i.e., where the frequency is less than the scan speed divided by the slit width). On the other hand, causes of blur include across slit tilt, chromatic aberration through the source bandwidth, and high-frequency vibration (i.e., where the frequency is greater than the scan speed divided by the slit width). Chromatic aberration and tilt induced blur are illustrated in FIGS. 2 and 3, respectively. In FIG. 2, chromatic aberration blur causes the different light frequencies from source 20 to focus at different planes 26b, 26c, 26d, and not on wafer surface 24. In FIG. 3, tilting of lens 22 at angle α from a line parallel to surface 24 causes multiple focal planes 26e, 26f, 26g at different angles or tilts of the image plane 28 from wafer surface 24 across slit 29. Thus, it would be desirable to be able to distinguish defocus from blur to optimize the lithographic patterning process.
U.S. application Ser. No. 10/771,684 by one of the instant inventors discloses a method for determining imaging and process parameter settings of a lithographic pattern imaging and processing system. The method correlates the dimensions of a first set of control patterns printed in a lithographic resist layer, measured at two or more locations on or within each pattern that correspond to different optimum focus settings, to the dose and focus settings of the pattern imaging system to produce dependencies. The method then measures the dimensions on subsequent sets of control patterns printed in a lithographic resist layer at two or more locations on or within each pattern, of which a minimum of two locations corresponding to different optimum focus settings match those measured in the first set, and subsequently determines the effective dose and defocus values associated with forming the subsequent sets of control patterns by comparing the dimensions at the matching locations with the correlated dependencies. However, the application discloses no method of determining blur error in control patterns, or the independent measurement and control of blur, defocus and dose error.